The amount of information transmitted in networks has increased because of the spread of internet technology and so on, and a large capacity transmission scheme is requested. As one of the schemes that increases transmission capacity, OFDM (Orthogonal Frequency Division Multiplexing) has been put into practical use. OFDM transmits data by using a plurality of subcarriers orthogonal to each other. Thus, OFDM may transmit a plurality of signals modulated in different modulation schemes.
However, since a plurality of modulated signals are multiplexed in OFDM, a large peak power may occur. That is to say, a peak-to-average power ratio (PAPR) may be high in OFDM.
When the PAPR in the OFDM signal is high, a transmitter requires a high-power amplifier with a wide dynamic range. However, high-power amplifiers with a wide dynamic range are expensive and have a low power efficiency. Note that when the dynamic range of the amplifier is narrow and the OFDM signal is amplified in the non-linear range, a waveform of the signal is distorted. In this case, communication quality deteriorates.
This problem (that is, that the PAPR in the OFDM signal is high) may be solved by, for example, a function to clip a peak of the OFDM signal. This “clipping” is realized by removing the peak power higher than a specified threshold. Note that a method for clipping a peak in the OFDM signal is described by, for example, [1] J. Armstrong, “New OFDM Peak-to-Average Power Reduction Scheme”, IEEE Vehicular Technology Conference, Amy 2001, Rhodes, Greece, and [2] A. Saul, Analysis of Peak Reduction in OFDM Systems Based on Recursive Clipping, Proc. of Int. OFDM-Workshop, Vol. 1, Hamburg, Germany, Sep, 24-25, 2003. In addition, WO 2008/129645 describes related art.
As described above, it is possible to reduce the PAPR in the OFDM signal by clipping a peak in the OFDM signal. However, if the peak in the OFDM signal is reduced too much, the quality of the signal deteriorates. In this case, a receiver sometimes cannot demodulate the signal correctly. Therefore, it is preferable to determine the threshold (hereinafter, this may be referred to as a “clipping level”) for clipping a peak of the OFDM signal according to a modulation scheme of the signal multiplexed in the OFDM signal.
However, the OFDM signal may transmit a plurality of modulated signals generated in different modulation schemes. When a plurality of modulated signals generated in different modulation schemes are multiplexed in the OFDM signal, a specified quality should be ensured for each modulation scheme. That is, it is requested that a quality of a signal be higher than the required level for each modulation scheme.
In order to ensure the quality of the signal for all modulation schemes, a clipping level that corresponds to a modulation scheme with the highest required quality may be used. When the quality of the signal is indicated by an Error Vector Magnitude (EVM), the clipping level is determined based on the modulation scheme with the smallest acceptable EVM. For example, when a QPSK signal, a 16QAM signal and a 64QAM signal are multiplexed in an OFDM signal, the clipping level is determined based on the acceptable EVM for the 64QAM.
However, if the clipping level corresponding to the modulation scheme with the smallest acceptable EVM is applied to all modulation schemes, a modulation scheme with the larger acceptable EVM has a margin in EVM. In other words, it is possible to further reduce the PAPR by using a lower clipping level for the modulation scheme with the larger acceptable EVM. Thus, it is considered that the PAPR in the OFDM signal is not sufficiently reduced in the prior art.